1. Field of the Invention
This invention relates to molten carbonate fuel cells and more particularly to cathode electrodes for molten carbonate fuel cells.
2. Description of the Prior Art
Fuel cells which use alkali-metal carbonates as the electrolyte are well known in the art and are generally referred to as molten carbonate fuel cells since the electrolyte is liquid at typical operating temperatures in the range of 550.degree. C. to 750.degree. C. The electrolyte is usually mixed with an inert particulate material which remains solid during cell operation to maintain spacing between the electrodes. The combination of electrolyte and inert is referred to as a tile at room temperature. Most molten carbonate fuel cell electrolytes are ternary or binary mixtures of, for example, lithium carbonate, potassium carbonate, and sodium carbonate. Molten carbonate electrolytes are discussed in greater detail in U.S. Pat. Nos. 3,998,939 and 4,079,171. Molten carbonate fuel cell systems in general are described in commonly owned U.S. Pat. Nos. 3,615,839 Thompson et al; 4,041,210 Van Dine; and 4,080,487 Reiser.
Anode electrodes for these cells may be, for example, nickel or cobalt. Prior art cathode electrodes for these cells are made of silver or nickel. The nickel oxidizes during cell operation, and it is the nickel oxide which provides the catalytic activity of the cathode. The cathode reaction is shown by the following equation: EQU , 1/2O.sub.2 +CO.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.= ( 1)
The anode reaction is shown by the following equation: EQU CO.sub.3.sup.= +H.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O+2e.sup.-( 2)
One function of the material from which the cathode is made is to catalyze the reduction of oxygen so as to accelerate the reaction of equation (1) to the right. Other desired properties of the cathode are high porosity, electronic conductivity, structural integrity, corrosion resistance, and stability in terms of its performance level. Silver is an excellent catalyst for the reduction of oxygen and initially performs very well; however, it has poor corrosion resistance and dissolves in the electrolyte. The dissolved material migrates toward the anode causing performance to deteriorate relatively quickly with time. Nickel oxide does not provide as good an initial performance as silver but it is significantly more corrosion resistant to the molten carbonate electrolytes. In manufacturing the electrode, nickel powder is initially formed into a porous plaque. Upon use in the fuel cell the nickel reacts with the electrolyte and the oxidant and oxidizes to nickel oxide. As it oxidizes the plaque tends to lose its structural integrity. Since the nickel oxide structure cannot be controlled, electrodes cannot be manufactured which will have optimum structural characteristics during cell operation.
It has been discussed by R. J. H. Voorhoeve in Chapter 5 of "Advanced Materials in Catalysis" edited by J. J. Burton and R. L. Garten, Academic Press, NY (1977), that lanthanum, strontium, cobalt, nickel and some other perovskites have properties making them suitable for use as cathode catalysts in both low temperature aqueous electrolyte cells and high temperature solid electrolyte cells. Examples of low temperature aqueous electrolytes are acids, such as phosphoric acid, and bases, such as potassium hydroxide. Solid oxide electrolytes are, for example, calcia doped zirconia or gadolinia doped ceria. Solid oxide cells are typically operated in the 1000.degree. C. range in order to get adequate ionic conductivity of the electrolyte. Solid oxide cells are discussed in U.S. Pat. No. 3,533,849 Mitoff. In that patent perovskites such as La.sub.0.6 Sr.sub.0.4 FeO.sub.3, LaCoO.sub.3, Co.sub.0.8 Sr.sub.0.2 LaO.sub.2 and others were tested as cathodes. All of them either performed poorly, had a short life, or degraded rapidly as discussed in column 2 and in Table I thereof. Solid electrolyte cells are also discussed in British Pat. No. 1,070,937 published June 7, 1967. In that patent lanthanum aluminate electrolyte is doped with the perovskite LaMnO.sub.3. The cell operates at about 400.degree. C. The LaMnO.sub.3 is supposed to make the surface of the electrolyte act like an electrode and thus eliminate the need for the electrode.
It is important to keep in mind that molten carbonate electrolytes provide a completely different type of chemical, thermal and corrosive environment than any of the foregoing types of cells. More specifically, the reactions being catalyzed are totally different.